Handbook of Electrical Installation Practice - BeKnowledge

normally located towards the outer perimeter of the complex and can be treated
as an independent sub-mains. After calculation of the area load in the sub-mains
vicinity a suitable MDA can be selected and the output requirements allowed for.
The supply cables to the sub-mains MDAs require careful selection and when the
route has been determined the appropriate cable size can be chosen. Factors governing
this selection are current rating of MDA, distance from supply incoming unit,
method of installation, and the type of protection. The voltage drop should not be
more than 4% of the declared voltage.
Table 5.5 details typical conductor and armour resistances for PVC/SWA/PVC
copper cables to BS 6346, and this information can be used to determine the
prospective fault levels at the sub-mains position.
Using the information in Tables 5.4 and 5.5, and assuming a power transformer
is situated adjacent to the main site protection, and a fault of negligible impedance,
the fault current is calculated as follows (Fig. 5.7):
2
[ ]
2
If = 230 ( R1 + R2 + R3) + ( X1+ X2)
Electricity on Construction Sites 113
where If = fault current
R1 = transformer resistance
R2 = phase conductor resistance
R3 = armour resistance (negligible reactance assumed)
X1 = transformer reactance
X2 = phase conductor reactance.
Table 5.5 Details of copper conductor cables to BS 6346. All values are
based on maximum conductor temperature (70°C).